1. Field of the Invention
The present invention relates to a polarizing optical system such as polarizing beam splitter and spatial light modulator for effecting polarizing modulations which utilizes an optical glass having an extremely small photoelastic constant C.
2. Related Background Art
In recent years, the utilization of a "polarizing characteristic", as one of the factors constituting optical information, has rapidly been developed in various fields such as the field of liquid crystal. Examples of an optical element for controlling polarized light, includes a polarized light-modulating type spatial light modulator (SLM) for spatially modulating polarized lights or a polarizing beam splitter for separating a P-polarized light and an S-polarized light from each others etc. Some apparatuses (such as projection-type display device) utilizing such an optical element have already been put to practical use.
Along with such development in the utilization of the polarizing characteristic, in an optical system utilizing polarized light, i.e., a polarizing optical system, the importance of high-precision control of the polarizing characteristic constituting optical information has been increased year by year. Based on the increase in the above-mentioned importance, it has earnestly been desired to further improve the precision or accuracy in the control of the polarizing characteristic.
Among various optical elements constituting a polarizing optical system (such as substrate and prism), it is usual to use a material having an optical isotropy especially for some optical elements which are required to retain the polarizing characteristic. The reason for this is that when an optical element comprising a material having an optical anisotropy is used, the phase difference (optical path difference) between the ordinary ray and the extraordinary ray perpendicular to the ordinary ray will be changed during their passage through such a material, with respect to light which has been transmitted by the optical element, and therefore the polarizing characteristic cannot be retained in such a case. In other words, even in a case where an optical element or component constituting a certain optical system has a performance of precisely controlling the polarized lights good characteristic cannot be obtained by the entire optical systems if the substrate or base material as another component constituting the optical system (which should retain the polarizing characteristic) impairs the polarizing characteristic.
In general, a glass which has sufficiently been subjected to annealing has an optical isotropy and also has various characteristics better than those of other materials in view of its durability, strengths transmittance, refractive index, cost, etc., and therefore such a glass is widely used for optical elements which should retain the polarizing characteristic. Particularly, borosilicate glass (e.g., a borosilicate glass mfd. by Schott Co., Germany, trade names "BK7") is inexpensive and excellent in durability, and also has little dispersion. Therefore, the borosilicate glass is widely used in many polarizing optical systems.
However, even when the above-mentioned conventional optical glass for polarizing optical system is used for the optical elements, a certain optical anisotropy based on a photoelastic effect can be induced in the optical element, under the application of a mechanical external stress or a thermal stress to the optical element. Accordingly, when the conventional optical glass is used for the optical element for a polarizing optical system the polarizing characteristic of optical information can be changed on the basis of the "induced optical anisotropy" as described above. Therefore, in such a case, it is difficult for the polarizing optical system to exhibit a desired performance.
It is considered that the mechanical external stress and the thermal stress as described above are developed mainly in the following situation.
Thus, it is considered that the "mechanical external stress" is mainly developed in a step of processing a glass (such as cutting the bonding or joining of the glass with another material, and film formation on the surface of a glass) or often a step of assembling a glass into an optical system (such as holding of the glass by a jig or holding device, and the adhesion of the glass to another member). In addition, it is considered that the "thermal stress" is developed by the production of heat in the interior of a glass (such as heat production based on the absorption of light energy), or the production of heat outside a glass (e.g., that based on heat production in a peripheral device). Furthers when a glass is caused to contact or is joined with another material having a thermal expansion coefficient different from that of the glass, it is considered that a stress is developed along with the above-mentioned production of heat.
As described above, when a polarizing optical system is constituted by using an optical element, it has been difficult to completely obviate the action of the mechanical external stress or the thermal stress. Accordingly, when the conventional optical glass for polarizing optical system is used for such an optical system, it is extremely difficult to avoid the induction of the optical anisotropy based on the above-mentioned mechanical external stress or thermal stress.